It is known to provide control arrangements in order to maintain a specific air/fuel ratio of the air/fuel mixture supplied to an internal combustion engine which obtain their control variable from an oxygen measuring probe mounted in the exhaust-gas system of the internal combustion engine. This control is generally superposed on a known mixture control arrangement by which the air/fuel mixture is roughly controlled in a preliminary manner. It is a condition for the smooth functioning of such control arrangement that the oxygen measuring probe works smoothly. In the case of the known oxygen measuring probes, operational readiness is only ensured after a certain operating temperature has been reached. For this reason, a mixture open-loop control must be provided for the cold start and the running up of the internal combustion engine in the case of probes which are heated up by the exhaust gases of the internal combustion engine. This mixture open-loop control is only replaced by the mixture closed-loop control when the probe temperature rising with the operating temperature of the internal combustion engine is reached.
The operational readiness of an oxygen probe with an ion-conducting solid-state device can be monitored in various ways. Thus, a monitoring arrangement is known from German Patent 2,301,354 in which voltage signals having variable potential and emitted by the oxygen measuring probe at short intervals determined by operating conditions are transmitted via a threshold switch to a timing element. As a consequence of these signals, the timing element holds a switch in a first switch position as long as the time interval of the voltage signals is shorter than the switching time of the timing element. In the other case, the switch is brought into a second switch position after this switching time has elapsed. By means of this switch, the operating mixture can then be influenced or a warning device can be switched on.
This arrangement has the disadvantage that it merely detects the total failure of the oxygen measuring probe when this probe no longer emits a signal with variable voltage because of a fault or at too low a temperature.
A method and an arrangement for monitoring the operational readiness of an oxygen measuring probe are presented in German Patent 2,608,245 which are already able to detect with certainty the operating conditions having reduced operational readiness of the oxygen measuring probe. In these cases, the mixture closed-loop control arrangement otherwise operated by means of the probe can be converted to a mixture open-loop control arrangement as early as possible. That is, as soon as the oxygen measuring probe emits a sufficiently high voltage signal for reliable operation of the closed-loop control arrangement.
This is effected by determining two voltage threshold values corresponding to the minimum and maximum voltage emitted by the probe when it is operationally ready. As soon as the increasing output voltage of the oxygen measuring probe resulting from the rising temperature exceeds these determined threshold values, this is detected by the comparator devices. The mixture open-loop control arrangement is switched off with insertion of a timing element and, when the signal change per time unit is less frequent, the mixture open-loop control arrangement is switched on again.
A method for recognizing the operational readiness of an oxygen measuring probe is known form the European patent application U.S. Pat. No. 4,742,808 wherein an exact measurement of the internal resistance of the oxygen measuring probe is made. A criterion for the operational readiness is given by the subsequent comparison with a predetermined value.
A method and an arrangement for controlling the temperature of an exhaust-gas probe are known from U.S. Pat. No. 4,419,190 which convert the temperature-dependent electrical resistance of the oxygen measuring probe into a temperature signal and then activate a heater device for heating the exhaust-gas probe. The operating temperature of the exhaust-gas probe can thereby be kept constant in a simple manner and without additional sensors and measurement leads.
In order that the exhaust-gas sensor may reach its operating temperature as quickly as possible and can subsequently also be kept at a predetermined temperature, it is prerequisite that the heater for the exhaust-gas probe, in the following called probe heater, be operationally ready.